C 语言中的最佳计时方法? ？

什么是高分辨率和可移植性的代码段计时的最佳方法？

/* Time from here */
ProcessIntenseFunction();
/* to here. */


printf("Time taken %d seconds %d milliseconds", sec, msec);

是否有一个标准库可以提供跨平台的解决方案？

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I use SDL_GetTicks from the SDL library.

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最佳答案

I think this should work:

#include <time.h>


clock_t start = clock(), diff;
ProcessIntenseFunction();
diff = clock() - start;


int msec = diff * 1000 / CLOCKS_PER_SEC;
printf("Time taken %d seconds %d milliseconds", msec/1000, msec%1000);
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gettimeofday will return time accurate to microseconds within the resolution of the system clock. You might also want to check out the High Res Timers project on SourceForge.

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gettimeofday() will probably do what you want.

If you're on Intel hardware, here's how to read the CPU real-time instruction counter. It will tell you the number of CPU cycles executed since the processor was booted. This is probably the finest-grained, lowest overhead counter you can get for performance measurement.

Note that this is the number of CPU cycles. On linux you can get the CPU speed from /proc/cpuinfo and divide to get the number of seconds. Converting this to a double is quite handy.

When I run this on my box, I get

11867927879484732
11867927879692217
it took this long to call printf: 207485

Here's the Intel developer's guide that gives tons of detail.

#include <stdio.h>
#include <stdint.h>


inline uint64_t rdtsc() {
uint32_t lo, hi;
__asm__ __volatile__ (
"xorl %%eax, %%eax\n"
"cpuid\n"
"rdtsc\n"
: "=a" (lo), "=d" (hi)
:
: "%ebx", "%ecx");
return (uint64_t)hi << 32 | lo;
}


main()
{
unsigned long long x;
unsigned long long y;
x = rdtsc();
printf("%lld\n",x);
y = rdtsc();
printf("%lld\n",y);
printf("it took this long to call printf: %lld\n",y-x);
}
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High resolution is relative... I was looking at the examples and they mostly cater for milliseconds. However for me it is important to measure microseconds. I have not seen a platform independant solution for microseconds and thought something like the code below will be usefull. I was timing on windows only for the time being and will most likely add a gettimeofday() implementation when doing the same on AIX/Linux.

    #ifdef WIN32
#ifndef PERFTIME
#include <windows.h>
#include <winbase.h>
#define PERFTIME_INIT unsigned __int64 freq;  QueryPerformanceFrequency((LARGE_INTEGER*)&freq); double timerFrequency = (1.0/freq);  unsigned __int64 startTime;  unsigned __int64 endTime;  double timeDifferenceInMilliseconds;
#define PERFTIME_START QueryPerformanceCounter((LARGE_INTEGER *)&startTime);
#define PERFTIME_END QueryPerformanceCounter((LARGE_INTEGER *)&endTime); timeDifferenceInMilliseconds = ((endTime-startTime) * timerFrequency);  printf("Timing %fms\n",timeDifferenceInMilliseconds);
#define PERFTIME(funct) {unsigned __int64 freq;  QueryPerformanceFrequency((LARGE_INTEGER*)&freq);  double timerFrequency = (1.0/freq);  unsigned __int64 startTime;  QueryPerformanceCounter((LARGE_INTEGER *)&startTime);  unsigned __int64 endTime;  funct; QueryPerformanceCounter((LARGE_INTEGER *)&endTime);  double timeDifferenceInMilliseconds = ((endTime-startTime) * timerFrequency);  printf("Timing %fms\n",timeDifferenceInMilliseconds);}
#endif
#else
//AIX/Linux gettimeofday() implementation here
#endif

Usage:

PERFTIME(ProcessIntenseFunction());


or


PERFTIME_INIT
PERFTIME_START
ProcessIntenseFunction()
PERFTIME_END
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If you don't want CPU time then I think what you're looking for is the timeval struct.

I use the below for calculating execution time:

int timeval_subtract(struct timeval *result,
struct timeval end,
struct timeval start)
{
if (start.tv_usec < end.tv_usec) {
int nsec = (end.tv_usec - start.tv_usec) / 1000000 + 1;
end.tv_usec -= 1000000 * nsec;
end.tv_sec += nsec;
}
if (start.tv_usec - end.tv_usec > 1000000) {
int nsec = (end.tv_usec - start.tv_usec) / 1000000;
end.tv_usec += 1000000 * nsec;
end.tv_sec -= nsec;
}


result->tv_sec = end.tv_sec - start.tv_sec;
result->tv_usec = end.tv_usec - start.tv_usec;


return end.tv_sec < start.tv_sec;
}


void set_exec_time(int end)
{
static struct timeval time_start;
struct timeval time_end;
struct timeval time_diff;


if (end) {
gettimeofday(&time_end, NULL);
if (timeval_subtract(&time_diff, time_end, time_start) == 0) {
if (end == 1)
printf("\nexec time: %1.2fs\n",
time_diff.tv_sec + (time_diff.tv_usec / 1000000.0f));
else if (end == 2)
printf("%1.2fs",
time_diff.tv_sec + (time_diff.tv_usec / 1000000.0f));
}
return;
}
gettimeofday(&time_start, NULL);
}


void start_exec_timer()
{
set_exec_time(0);
}


void print_exec_timer()
{
set_exec_time(1);
}

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